Alternative compressors used in refrigeration systems have a connecting rod mounted, at one side, to an eccentric pin of a crankshaft which is journaled in a bearing block and, on the other side, to an alternative piston within a cylinder, which is orthogonal to the geometrical axis of the eccentric pin, between a lower dead point condition and an upper dead point condition, respectively, defined by a maximum and a minimum linear spacing between a front-end face of the piston and a valve plate, seated and fixed against a front-end of the cylinder and defining the inner face of a head assembly which forms suction and discharge chambers of the compressor.
In these constructions, at the upper dead point condition, the piston should be within a certain distance from a confronting face of the valves plate, defining, in this region, a portion of the compression chamber and, consequently, a portion of the minimum dead volume, previously determined to avoid that, during normal operation of the compressor, any impacts of the piston against the valve plate occur.
As to avoid compromising the compressor performance, the portion of the compression chamber in which is defined a corresponding portion of the dead volume, originated in the adjacent region to the valve plate of the head, it must have a small value as possible to avoid the loss of efficiency of the compressor.
Due to the reduced size of the components and the small space for mounting the piston-connecting rod-eccentric pin assembly, different alternatives have been proposed for constructing a connecting rod, aiming to facilitate mounting of this assembly in the compressor. Among the many alternatives, it can be cited those using connecting rod with two-parts, as occurs in solutions U.S. Pat. Nos. 4,930,405 and 6,551,067, of the same Applicant. Also, solutions of two-parts connecting rod as described in documents KR2008-0017214 and JP5584880 are known. The construction of the two-parts connecting rod has the advantage of facilitating the mounting of the connecting rod assembly to the parts of the eccentric pin and the piston in the compressor.
Although facilitating the mounting in the compressor, the two-parts connecting rod require that its final mounting is made to provide:                the correct axial alignment (or longitudinal parallelism) between the geometrical axis of the larger and the smaller eye (or perpendicularism between the major geometrical axis of the larger eye and the two-parts connecting rod in the case of a spherical joint);        the correct cross-parallelism (rotational alignment) between the axis of the larger and the smaller eye, and further;        the correct total connecting rod length (or relative axial positioning between the geometrical axis of the larger and the smaller eye, wherein the axial direction is relative to the connecting stem between the two eyes) to produce a corresponding portion of dead volume as small as possible for each compressor to be mounted. For avoiding the specification of extremely low tolerances and expensive in each compressor being mounted, it is commonly used the artifice of providing different groups of sealing gasket to be positioned between the front-end face of the cylinder and the confronting face of the valves plate, for providing the final adjustment of the dead volume. However, the use of different types and groups of the gasket to achieve the correct dimensioning of the compression chamber and, consequently, the respective portion of dead volume in each compressor unit being manufactured, is highly undesirable, since it requires cumbersome operations of measuring the protrusion of the piston, the provision of different groups of gasket and further, the selection of said elements for mounting each compressor unit.        
Some of the solutions of the prior art which use the two-parts connecting rod do not produce a final attachment between the two stem portions, capable of allowing a continuous variation of the total final connecting rod length, specifically and properly sized to provide the same pattern of portion of dead volume in the compression chamber, regardless of the use or not of a single group of sealing gasket between the front-end face of the cylinder and the valve plate. This occurs with the solutions of the type described in documents U.S. Pat. No. 4,930,405, KR2008-0017214 and JP55-84880.
In another prior art solution, which allows a continuous adjustment by telescopic engagement, the two portions of the connecting rod, as described in document U.S. Pat. No. 6,551,067B1, results from the fact that the telescopic engagement is made with an interference in necessary degree and sufficient to hold the two stem portions in a telescopic positioning determined by the axial compression of the two parts, generally obtained by seating of the front-end face of the piston against a mounting shim, having predetermined sizing and which is held seated against the front-end face of the cylinder, when the crankshaft is rotated until it reaches the upper dead point of the piston.
A major problem which has limited and even prevented the effective use of this solution with two-parts telescopic connecting rod, as described in document U.S. Pat. No. 6,551,067B1, results from the difficulty and even the impracticality to produce, in the construction of the two portions of the stem, a dimensional accuracy that leads to a degree of mechanical interference in the telescopic engagement, sufficient to ensure a proper axial force range, defined:
(a) By a minimum axial force of insertion, in order to keep the two portions of the stem in the correct telescopic positioning, defined at the upper dead point of the piston, until the final fixation is produced between the two portions;
(b) By a maximum axial force of insertion which is insufficient for the compressive forces, for adjustment of the telescopic engagement provided between the eccentric pin of the crankshaft and the mounting shim, do not produce deformations in the assembly of block-cylinder-connecting rod-axis capable of impairing the proper sizing of the dead volume to be obtained in each compressor, using a single group or minimized variation for the sealing gasket to be provided between the valve plate and the front-end face of the cylinder.
Due to the difficulties in obtaining, in an economically feasible manner, the suitable degree of mechanical interference between the two stem portions, to be telescopically engaged together, it is common to provide a degree of insufficient interference to ensure the maintenance of the telescopic positioning defined at the upper dead point of the piston, when the crankshaft is rotated to bring the piston-connecting rod assembly to the lower dead point, facilitating the final fixation of the two portions by different solutions, such as by welding, brazing, adhesive, and the like. On the other hand, when the degree of interference is excessive, the compressive force between the two portions of the connecting rod, for the adjustment of the telescopic positioning, produces elastic deformations in the involved parts, capable of producing deviations of sizing of the desired dead volume, in sufficient degree to require the resizing/replacement of the sealing gasket, for obtaining a compression chamber portion that defines the desired respective portion of dead volume.
Due to the difficulties of obtaining a suitable degree of mechanical interference in the telescopic engagement and the dimensional variations present in the components of the compressor and which can modify the relative position between the parts of the eccentric pin and the piston, generally, the mounting of the connecting rod to the latter determines a length that differs from that optimal desired and which requires final settings by using different groups of sealing gasket between the valve plate and the front-end face of the cylinder.